Fuel assemblies of water-cooled nuclear reactors and in particular pressurized water nuclear reactors comprise a framework in which are introduced fuel rods constituted by a sheath enclosing a nuclear fuel material, such as uranium or plutonium oxide in the form of sintered pellets.
The sheath constructed from a tube of zirconium alloy must have good resistance to corrosion under the effect of the primary fluid circulating in contact with the outer surface of the sheath.
For forming the sheath of fuel rods of assemblies of water-cooled reactors there is usually employed alloy including principally 1.2 to 1.7% tin, 0.18 to 0.24% iron and 0.07 to 0.13% chromium or a zirconium alloy including 1.2 to 1.7% tin, 0.07 to 0.2% iron, 0.05 to 0.15% chromium and 0.03 to 0.08% nickel.
In order to improve the performances as concerns corrosion under irradiation of the sheaths of fuel rods in the environment of the nuclear reactor in operation and thereby prolong the life of the fuel assemblies in the core, there have been proposed modifications or adjustments in the composition of the aforementioned zirconium alloys or replacements by alloys including elements such as vanadium, niobium or copper.
However, these alloys do not present decisive advantages over currently employed alloys whose composition is mentioned hereinbefore, in that their mechanical characteristics and in particular their hardness and their resistance to wear are usually lower than those of the currently employed compositions.
In addition to the corrosion of their outer surface exposed to the cooling fluid of the nuclear reactor, the sheaths undergo internal corrosion due in particular to the interaction between the fuel pellets and the inner surface of the sheath. In order to reduce this internal corrosion, it has been proposed to deposit a layer of insulation between the pellets and the sheath.
It has also been proposed in patent application EP-A-0,212,351 to construct the sheath in the form of a duplex tube comprising a tubular inner layer of zirconium alloy of the conventional type such as described hereinbefore and a surface layer improving the resistance to corrosion of the sheath composed of zirconium alloy different from the alloy constituting the inner layer and including iron and at least one of the elements vanadium, platinum and copper. This surface layer, whose thickness represents 1 to 20% of the total thickness of the wall of the sheath, may be produced by extrusion of a blank constituted by an inner tube of zirconium alloy of conventional composition on which is mounted an outer tube having the composition of the surface layer.
The sheath is then rolled in a step-by-step rolling mill down to its definitive diameter.
The fuel rods, whose sheath is constituted by a duplex tube, present a generalized outer corrosion resistance in the environment of the nuclear reactor which is distinctly improved. However, the hardness and the resistance to wear of the surface layer are lower than those of sheaths of alloy of conventional type.
Consequently, when loading a fuel assembly with new rods or when replacing rods in a worn fuel assembly, the sheaths of the rods undergo, on their outer surface, increased wear which may result in defects and increased local corrosion.